Sewing threads composed of a multifilament yarn have been widely used because of the excellence in mightiness, wear resistance, and uniformity in comparison with sewing threads composed of short fibers. Among them, several proposals have been made in that sewability is improved by processing filaments with fluid into a filament textured yarn, such as a spun yarn having loops.
In Japanese Unexamined Patent Application Publication No. 62-257434, a thread is proposed, which is processed with a fluid turbulent. processor so as to have difference in yarn length for forming loops. However, this thread is expensive in processing because of the. fluid turbulent processor, and also because of many uneven fine loops, and there have been problems of low process-passage efficiency, such as unreeling failure produced in a twisting process and a sewing process, and reduction in mightiness.
This is because one filament of single. fiber forms a torsional loop so as to stiffly protrude from a yarn surface, so that the loop is difficult to contribute to yarn strength, and produces strong fastener effect when the yarn is unreeled. This. becomes a fatal defect so that there is no prospect of a practical application as a sewing thread.
Japanese Unexamined Patent Application Publication No. 5-106134 discloses a composite sewing machine thread in that the ductility difference between a spontaneous ductile filament yarn and a non-spontaneous ductile filament yarn forms loops and slack. However, the fineness of the loop and slack is small and the number of the loops is scarcely counted, so that accompanying airflow effect and annealing effect of needle heat are small, and the thread is insufficient to have high-speed sewability.
on the other hand, a conventional cotton sewing thread having excellent sewability has been widely used as home sewing threads and industrial sewing threads. However, the mightiness is small and dyeing fastness is also insufficient. Moreover, this thread has defects, such as being susceptible to dimensional change and insufficient seam appearance. On the other hand, a polyester spun sewing-thread enjoying a large market share of sewing threads has the sewability similar to the cotton, and also is excellent in mightiness, dyeing fastness, and dimensional stability. However, since it is produced by a spinning process, there are problems in length control and quality control, such as large variations in gauge and mightiness, and a large number of knots, so that the thread is difficult to be used for sewing high-quality cloth because of its low appearance quality.
A sewing thread produced from filament yarns of silk, polyester, and polyamide has been widely used as a sewing thread in order correct the defects of cotton and polyester spun sewing threads. A conventional filament sewing thread is produced by providing upper-twist to a plurality of aligned single yarns provided with under-twist, and its physical characteristics can be obtained in accordance with the fineness of the single yarn and the overall fiber fineness of the sewing thread, so that it has stable quality.
However, with a conventional filament sewing-thread, in the general sewing, i.e., the forward sewing with a lock stitch sewing machine, the thread can be favorably sewn, whereas, in the backward sewing, since a force is applied in a direction canceling the upper-twist, twist crack and broken yarn occur, so that the thread has a fatal defect that the thread cannot be applied to automatic machine sewing, in which the backward sewing is frequently incorporated. In high-speed sewing with an industrial sewing machine, since the frictional resistance on the side face of the filament yarn is large, broken yarns are liable to be generated by needle heat and the penetration resistance against cloth. This corresponds to no suppressing effect on needle heat temperature mentioned above.
Then, several sewing threads, in which the problems of conventional spun sewing threads and filament sewing threads are solved, have been proposed. A core-spun sewing thread having a sheath-core structure of a filament yarn and a spun yarn has been proposed in Japanese. Examined Patent Application Publication No. 63-3977; a sewing thread with sewability improved by union twisting of a spun yarn and a filament yarn has been proposed in Japanese Unexamined Patent Application Publication No. 2-33341.
However, these sewing threads also have knots and variations in gauge since it is produced by a spinning process as mentioned above, containing a defect that broken yarns are liable to be generated in sewing.
In order to apply fluffiness and bulkiness like in a spun yarn to a synthetic-fiber multifilament yarn, several threads processed by fluffing filament yarns are proposed. A fluffing method has been proposed in Japanese Unexamined Patent Application Publication No. 3-64546 and No. 8-337937 in that after under-twist or upper-twist is provided, a multifilament yarn is looped around a guide, and the yarn approaching the guide and the way back yarn are crossed to each other so as to apply the fluffiness to both the yarn by bearing down on them. However, in the sewing threads obtained by these methods, although they have the fluffiness like a spun filament, a load due to the crossing process to multifilament yarns is large for the sewing thread, so that the backward sewability is especially low and the yarn strength is insufficient.